Description
The invention relates to an integrated semiconductor device including at least one optoelectronic switching element, this element comprising:
two rectilinear monomode optical guides crossing each other at a given angle composed of at least one heterostructure of III-V material, which comprises a substrate S of a confinement material and a guiding layer C.sub.G as well as a guiding strip RB;
a p-n junction formed in the crossing region asymmetrically with respect to the bi-secting longitudinal plane of the crossing angle.
The invention is used in the formation of optical switching matrices of N guides in N guides for optoelectronic and telecommunication systems.
The invention further relates to a switching matrix formed by means of these elements and to a method of manufacturing a semiconductor device including such a matrix.
Such a switching element is known from U.S. Pat. No. 4,784,451. This document describes various switching elements of the charge carrier injection type, whose operation is improved by the presence of a current limiting structure. These switching elements comprise two wave guide sections enclosing a given angle with each other. These guides are integrated on a substrate and are formed from a guiding layer disposed between two confinement layers, which consist of a material having a greater forbidden band width and a lower refractive index. One of the confinement layers has one conductivity type and the guiding layer has the opposite conductivity type so as to form a p-n junction. On the other hand, Zn ions are implanted into each of the confinement layers to form the current-limiting structure. This structure limits a zone in the guiding layer in which the index is changed by the injection of the charge carriers.
In an embodiment, the switching element has, viewed from above, the form of a Y. The p-n junction is formed through a rectilinear guide, at one end of which the input is formed, while at its other end the output is formed in the passive mode, the output being obtained by a second branch of the Y in the active mode. In the latter mode, by change of index in the zone of the guiding layer, the light entering at the end designated as input of the rectilinear guide is reflected towards the second branch of the Y.
One of the electrodes controlling the switching operation is disposed asymmetrically at the surface of the Y, one of its edges being aligned with respect to the bisectrix of the angle of the two output branches. This electrode does not project beyond the width of the guides.
The current-limiting structure is such that it has two zones implanted at the upper part of the substrate and mutually separated by a distance about equal to the transverse dimension of the upper electrode and that it moreover has a zone implanted into the upper confinement layer and located substantially under the upper electrode. The lower electrode is disposed on the opposite surface of the substrate, the latter being conducting. The light is guided due to the fact that a mesa is formed at the surface of the substrate including the guiding layer and the upper layers.
In another embodiment described in the same aforementioned document, the switching element has the form of an X. The upper electrode has the form of a strip arranged parallel to the bisectrix of the small joining angle of the branches of the X. In this case, the substrate is provided at its surface with a layer of a conductivity type opposite to that of the guiding layer having the form of a strip disposed along the same axis as the upper electrode, i.e. parallel to the bisectrix of the joining angle of the brancehs of the X. Since said layer is at one level with the upper part of the substrate, electrodes of a conductivity type opposite to that of the upper electrode are disposed at each end of the said layer. A region implanted by means of Zn ions is disposed substantially under the upper electrode in the upper confinement layer. The upper electrode has a length smaller than that of the diagonal of the junction. It is disposed symmetrically on the element in the form of an X and its lateral dimension is not negligible. The lower part of the current-limiting structure in this case does not include regions implanted by means of Zn ions in the substrate, which is semi-insulating, due to the presence of the strip of a first conductivity type provided with the electrodes at each of its ends.
These structures mainly have the disadvantage that the reflecting surface formed by the region of modified index due to the injection of charge carriers by means of the p-n junction does not exhibit sufficiently large dimensions to receive all the incident beams and to reflect them. This results in the leakage of an evanescent wave at each end of the central region of the switching element, especially in the active mode.
Moreover, the different regions of the current-limiting structure are only approximately aligned due to the fact that they are formed by implantation of Zn ions at two levels. By this method, it is very difficult to align the edge of a second region at an upper level. The reflecting surface is therefore only approximately disposed.
A disadvantage of the known switching elements for the present application in telecommunication is that they are not monomode elements. More particularly, even if the dimensions of these elements are transposed to obtain the propagation of a monomode wave in the input guide(s), the arrangement of the upper electrode in the element in the form of an X admits the excitation of complicated modes. In fact, the reflecting surfaces do not coincide with the bisectrix of the joining angle, which is optically unfavourable for the reflection. Losses in the active mode also result therefrom.
On the other hand, the optical guides of the known devices are composed of a guiding layer disposed in a mesa between two confinement layers. Such a structure is subjected to losses by lateral diffusion through the walls, which are very substantial.
It should further be noted that the adoption of a substrate of the n-type is not only a disadvantage for the manufacture of a current-limiting structure, but also for the manufacture of optoelectronic devices in manufacturing synergy with that of other (active integrated) elements, such as field effect transistors.
The known devices are therefore anything but optimized both in the active and in the passive mode.
It will be taken into account that for applications in the field of telecommunication it must be possible to realize switching matrices operating at the wavelengths of 1.3 or 1.55 .mu.m, transporting monomode signals and having a large number of switching elements. Therefore, losses that may seem small when considering a single switching element become redhibitory when a whole matrix is utilized.